Crystalline materials have the highest mechanical Q's of all known materials. Their elastic properties are very stable and repeatable. Crystalline materials are thus normally used for deformation type applications wherein stability and repeatability are desired. One of the best crystalline materials insofar as stability and repeatability are concerned and most inexpensive as to cost is alpha quartz. This material has been used as a vibrating element for frequency control because of its stability and repeatability since at least the 1930's.
Most methods of measuring pressures involve the deformation of some type of material. Two general criteria are applied to such pressure measuring devices: the quality of the deforming material, and the inherent accuracy limits of the means and method by which the pressure measurement is made. In addition, the practicality of a pressure measuring means is also of primary importance. Capacitance pressure measuring devices offer a simple structure and convenient method of pressure measurement and thus comprise practical pressure measuring means. Capacitance pressure measuring devices normally consist of two parallel plates which comprise the plates of a capacitor, at least one of which plates deflects in response to pressure difference thereacross to vary the capacitance so that capacitance is related to pressure difference. Capacitance pressure measuring devices having a crystalline quartz deflecting plate are potentially very accurate because the quality of crystalline deforming material in the capacitance pressure measuring means will be excellent.